We propose to develop a miniature real-time volumetric ultrasound imaging system employing two-dimensional (2D) capacitive micromachined ultrasonic transducer (CMUT) arrays. We will design and implement a miniature probe fitting into the tip of an endoscope that will be connected to an external beamforming unit using a small number of cables. The probe will contain a 2D CMUT array and integrated circuits to handle the data acquisition and front-end pre-processing. A small-sized active phased subarray will be multiplexed over a large transducer array in consecutive firing steps to allow for real-time data acquisition. The acquired data will be transferred to the external unit and processed to form real-time pyramidal volumetric images. CMUT technology provides excellent element sensitivity, large temporal bandwidth, and potential for integrating the transducer with front-end electronic circuitry. The image reconstruction algorithm is based on phased subarray processing which allows for a low-cost front-end architecture while providing high-quality real-time images. The combination of CMUT technology and phased subarray (PSA) imaging will enable us to realize a small ultrasound scanner where the size and cost of the front-end circuitry is critical. We have achieved the following preliminary results: (1) Identified several image-guided surgical applications for cancer treatment, (2) specified the probe parameters based on physician input, (3) fabricated 128-element 1D CMUT array and (4) used it to form a high-quality 2D image, (4) fabricated a prototype 16x16-element 2D CMUT array, (5) developed PSA theory for 2D and 3D imaging, (6) tested 2D PSA theory with simulations and experimental data, and (7) shown that the system specs allow for real-time 3D ultrasound imaging. We propose to take the next major developmental step and demonstrate real-time 3D imaging from a miniature probe by accomplishing the following specific aims: (1) form preliminary 3D images using the prototype array and existing acquisition system, (2) design and fabricate new 16x16 2D CMUT arrays, (3) implement real-time beamforming and visualization, (4) design and build integrated circuits for the front-end probe electronics, (5) perform final integration into a fully function real-time 3D ultrasound system, and (6) perform preliminary clinical evaluation.